A CT image for clinical diagnose is obtained based on data received by a detector. During the imaging process, it is generally assumed that, position of the focal point is fixed. In reality, the focal point position of an X-ray tube may shift due to the change of thermal capacity of the X-ray tube. The shift or offset of the focal point position may bring artifacts on the CT image which may reduce the image quality and influence the results of diagnosis. However, the focal point position is difficult to be measured and the offset of the focal point position is difficult to be corrected.